FIG. 1 of the accompanying drawings illustrates the stack structure of a typical liquid crystal display (LCD) module of small size, for example for a mobile phone or PDA device. The display comprises a flat transmissive spatial light modulator (SLM) in the form of an LCD panel having input and output polarisers on its bottom and top sides. The rest of the structure is generally regarded as the backlight system, as follows. A light source (for example an LED or Laser) emits light, which is coupled into a light guide and distributed across the back of the display by way of total internal reflection (TIR) in such a way that if no scattering structures were present the light would travel until it reached the end of the light guide. Within the light guide there are multiple scattering structures that extract the light from the light guide to illuminate the LCD panel by disrupting the TIR conditions at the surface of the light guide on which they are located, hence allowing the light to pass through an external medium (e.g., the air-lightguide interface). These scattering features may be located on either the top or bottom major lightguide surfaces. The density of the light scattering features may increase with distance from the light source to maintain a uniform rate of extraction of the light along the length of the lightguide. As light is extracted both down and up from the lightguide, a reflecting film is placed beneath the lightguide to improve the efficiency of the backlight. There are also some optical films between the lightguide and the LCD panel, placed to give better illumination uniformity over the display area and to enhance brightness within a given viewing angle range. These films typically consist of diffuser layers and prism films that enhance the central brightness of the backlight. The form of these structures are well known in the prior art and will not be discussed further here.
The features that extract the light can take many forms, and the particular form that the extraction takes can determine the angular profile of the lightguide emission, which then can be diffused or utilized in some manner. For example, in backlights that require the lightguide to produce collimated emission, the form of the emission will be very sensitive to the range of angles of the light in the lightguide. Other potential extraction, such as sub-wavelength, diffractive or holographic features, will also be dependent on the angle range for the quality of their extraction profile.
Another potential backlight is one that has wedge-type extraction that produces a collimated beam to enter a lens array to control direction on bending. The limiting factor in the minimum bend radius for a given lens pitch is the range of angles the lightguide emits, especially in a vertical direction. This is dependent on the range of angles that exists in the lightguide.
U.S. patent application Ser. No. 12/331,901 relates to a method to control the direction of the light in a lightguide. The method involves triangular shaped features positioned substantially parallel with the direction of the light coupled into the lightguide. The patent discloses a method for directing the light in a horizontal direction to reduce mixing region and more efficiently distributing light in a lightguide of unusual shape.
WO2008/047292 (Philips Electronics) describes a funnel-shaped reflector which collimates light in both viewing directions to enhance LED brightness. It is mainly aimed at general lighting applications and funnel has to be long for efficient collimation.
WO2007/144805 (Philips Electronics) describes a collimating and color mixing arrangement encapsulated to the LED chip. It comprises a lens placed in front of the LEDs and triangular features made of dichroic film, which enhance the overall LED brightness.
Both of the above patents relate to improving relative collimation between colour LEDs, rather than controlling the collimation itself.
US 2007/0133097 (Brilliant Film LLC) describes a set of films with concave, lens-type, refraction structures for collimating light out-coupled from lightguide in both viewing directions. Design concerns controlling light extracted from the lightguide.
US 2007/0058259 (Samsung Electronics) describes a set of films with BEF like structures for collimating light out-coupled from lightguide in both viewing directions. Design also concerns controlling light extracted from the lightguide.
US 2008/0062686 (Philips Electronics) describes an illumination system comprised of a light-collimation section, a light-mixing section and a light-shaping diffuser. This set of components is attached to the front of a lightguide to provide with uniform light, eliminating spatial and color mixing regions. Since it is an external component it adds an extra assembly step and also can be relatively expensive.
US 2008/0110358 (Xiper Innovations INC.) describes a collimating bar made by a Fresnel type, cylindrical, aspherical lens. Alignment between LEDs-Fresnel lenses is required in this type of systems making fabrication more complex. The sheet also needs some controlled separation from the light source.
US 2007/0247872 (Samsung Electronics) describes a collimating bar made of a series of triangular structures arranged horizontally along the lightguide input. Light is collimated in horizontal direction only.
US 2005/0073756 (P. Poulsen) describes a light collimator which comprises an array of elongated reflective channels. The light is then collimated by another film. The features here act as extraction features and do not reduce the spread of angles in the lightguide vertically.